For manufacturing semiconductor devices, such as integrated circuits and light emitting diodes, LEDs, it has long been a practice to employ chemical vapor deposition, CVD, as well as epitaxial processes for depositing various materials on substrates at high temperatures as part of the process of making these semiconductor devices. At these high temperatures a substrate holder such as a susceptor is used for supporting the substrate or substrates upon which the material is to be deposited by the well known processes of CVD and epitaxy. These susceptors are often heated by induction, by making use of the susceptance of the substrate holder material there made of, which is often graphite.
The susceptors used for these and other semiconductor manufacturing processes are know, for example from U.S. Pat. No. 3,980,854 and U.S. Pat. No. 4,047,496.
Examples of semiconductor production processes wherein susceptors are used are diffusion and oxidation processes and CVD processes for depositing polysilicon and dielectric layers, such as SiO2, Si3N4, SiOxNy and conductive layers such as WSix, TiN, TaN en TaO. Metalorganic Chemical Vapour Deposition, MOCVD, is another semiconductor production process wherein susceptors are used. Because, amongst others, the technique of MOCVD is preferred for the formation of devices incorporating thermodynamically metastable alloys, it has become a major process in the manufacturing of for example LEDs and solar cells.
As well as in MOCVD as in epitaxial processes substrates are supported by wafer carriers or susceptors (also called substrate holders). These supports are often heated by induction. When using induction the susceptance of the base material of the susceptor, which is often graphite, is used for heating the substrate by thermal contact with and/or radiation from the heated susceptor.
Most of the substrate holders used today are used in either vertical or horizontal systems wherein the substrate holders of the vertical type are known as a barrel type susceptor. In either case, there are susceptors which are configured to support (or hold) a multiplicity of smaller substrates, for simultaneously depositing materials on the multiplicity of substrates. For this type of simultaneously depositing of materials it is, like single substrate holders, difficult to produce semiconductor devices with a high and stable quality standard.
There are a number of technical difficulties in the process of manufacturing high quality semiconductor devices and especially in the process of MOCVD and in Epitaxial processes. A major factor of quality lies in stable process parameters. With an increasing demand for high quality semiconductor devices with low tolerance levels and an increasing demand for a high yield on the production process more stable process parameters are needed.
A major factor in unstable process parameters is the control of temperature which is critical at the elevated temperatures needed for proper deposition. It is difficult to control these temperatures within the critical tolerances at all the desired locations within the reaction chambers. It is know that due to the susceptor and the heater configuration, such as, an induction coil, various positions of the susceptor may adopt different temperatures. The wafer temperature is influenced by the contact area with the susceptor and distance to the susceptor. The temperature differences result in different deposition layer thicknesses, or in the case of MOCVD also in layer composition, from one substrate to another, and even in non uniform individual substrates. As a result of these different thicknesses a quality drop of the end products will occur and production yield will drop.
To a certain amount the temperature differences can be compensated by a difference in the design and shape of the susceptors. Most susceptors for example are provided with a recess for the placement of the substrate. At this recess the susceptor is of a different thickness, resulting in unwanted temperature fluctuations. It is known, for example from WO 2003 069029 A1 to provide indentations in susceptors to overcome and compensate for these temperature differences as a result of the different thicknesses. These general design rules however are only unique for a certain type of susceptor in a specific inductive reactor system.
Temperature differences within the susceptor however do not only occur as a result of non uniform thicknesses. Contamination by deposition of non-susceptance material on the susceptor and/or substrate, which may be occur during loading and unloading, lead to deficiency and therefore unwanted temperature differences. Furthermore the inhomogene properties of the base material, mostly graphite, of the susceptor also lead to temperature differences.
To compensate for these temperature differences it is commonly known to rotate the susceptor in the induction field, thereby, partially, compensating the differences in the induction field. Temperature differences in the susceptor due to the inhomogeneity of the base material and non uniformity of the thickness of individual susceptors are not compensated. It is also commonly known to rotate the wafers with respect to the susceptor by use of a small substrate holder, often referred to as a planetenscheibe. The small substrate holder is placed in the recesses of the susceptor and rotates apart from the susceptor. On this small substrate holder the wafer is deposited. Because of the second rotation of the small substrate holder a further compensation of the temperature differences is obtained. These small compensations however do not result in optimal temperature uniformity and therefore non-uniform deposition of the deposition materials of the wafer still exists.
With the increasing demand for high quality manufactured semiconductor devices and an increasing demand for a high production yield the need for improved temperature control increases.